which is a convex function with respect to (w.r.t.) \((d_{i1}, d_{i2})\)  since exponential function is convex, \(a_{i1}\ge 0\)  and \(a_{i2}\ge a_{i1}\)  according to (REF ) and \(h_{i1}\ge h_{i2}\) .
Based on [1]}, the perspective of \(u(x)\)  is the function \(v(x,y)\)  defined by
\(v(x,y)=tu(x/y), {\textbf {dom}}\:v= \lbrace (x,y)|x/y\in {\textbf {dom}}\:u, y>0\rbrace .\) 
